The invention relates to microporous separators and in particular to improved separators for use in energy storage cells, particularly lead acid batteries.
A conventional flooded lead-acid cell produces stoichiometric quantities of hydrogen and oxygen during the recharging process. Hydrogen and oxygen gases generated as a result of the electrolytic decomposition of the water in the cell eventually lead to significant water loss and the need for cell maintenance. Calcium-based lead grid alloys have reduced the water loss problem but have not eliminated such water loss. The inherent gassing problem associated with lead-acid cells historically prevented conventional flooded lead-acid cells from being operated as a xe2x80x9csealedxe2x80x9d cell like other battery systems such as nickel cadmium.
Sealed lead-acid cells are based on gelled electrolyte technology (Gel-Cell). These cells utilize a jelly-like mixture of sulfuric acid/fumed silica, a leaf separator between the plates of the cells made of polyethylene, polyvinylchloride and the like and use a low pressure, unidirectional relief valve to produce internal gas recombination known as a xe2x80x9crecombinant cell designxe2x80x9d. The key motivation for a recombinant cell design is to produce a cell with liquid electrolyte in the plates and voids containing semi-solid electrolyte between the plates. These voids or xe2x80x9ccracksxe2x80x9d are believed to enhance gas recombination by serving as pathways for gas movement from the positive plate to the negative plate. In this way a cell plate ratio is established so that the positive plates become fully charged before the negative plates. The oxygen produced at the fully charged positive plate fills the voids in the gelled electrolyte and ultimately reaches the surface of the negative plate. The net effect is to inhibit hydrogen evolution through its conversion or xe2x80x9crecombinationxe2x80x9d with oxygen to form water. Cells designed with this technology showed excellent life under continuous float charge conditions and have found many applications in emergency lighting and standby power systems. A significant disadvantage of the recombinant cell technology relates to the absence of acceptable high rate discharge performance of the cells due to the relatively high electrical resistance of the electrolyte/separator system.
An improvement in sealed lead-acid cell is a xe2x80x9cstarvedxe2x80x9d cell utilizing nonwoven glass mat separators as described in U.S. Pat. No. 3,862,861 to McClelland et al. Starved energy cells operate on an oxygen recombination cycle, as does the Gel-Cell, but offer higher rate performance due to the lower electrical resistance of the nonwoven glass mat separator.
The glass mat separators are fabricated from 100 wt.% glass micro fibers similar to those used in the manufacture of fiberglass insulation. The fibers are formed into a binderless, nonwoven mat or felt with the use of wet-laid papermaking technology and equipment. During the manufacture of the cells, the glass mat separators are compressed between the positive and negative plates of the lead-acid cell. The glass mat separators have a high rate of wetting and therefore readily absorb and retain the liquid sulfuric acid electrolyte between the battery plates. The cells are designed so that less than 100% of the separator pore volume is filled with electrolyte (i.e., xe2x80x9cstarvedxe2x80x9d) and in this manner the separators facilitate gas movement and facilitate oxygen and hydrogen recombination. A disadvantage of a cell of this design is the relatively high porosity of the separators which tends to cause poor dendrite protection and extremely high recombination rates. At higher levels of recombination, thermal runaway and negative self-discharge become insidious problems. Also, glass mat separators tend to be difficult to handle and easily torn or punctured during battery cell assembly
To a certain degree, conventional flooded lead-acid cells exhibit gas recombination. This is due to the fact that the gaseous oxygen is slightly soluble in the electrolyte and therefore is able to reach the surface of the negative plate via diffusion and convection. The degree of oxygen recombination in flooded cells is dependent on the type of separator used in the cell. For example, measured recombination rates are higher in cells employing microporous polyethylene separators than in cells using microporous rubber-based separators. A possible explanation for the rate difference between a polyethylene separator and a rubber-based separator is that polyethylene separators are more highly porous materials than the rubber-based separators leading to increased oxygen migration through the electrolyte. Another possible explanation is that the oxygen recombination reaction is partially inhibited by the presence of soluble compounds in the microporous rubber-based separators which are absorbed on the surface of the negative plates of the cell thereby affecting the surface chemistry of the negative plate during a recharge. Accordingly, there continues to be a need for an improved energy cell which exhibits improved high rate discharge performance and lower electrical resistance.
With regard to the above and other objects and advantages, the invention provides a separator for an energy cell which includes a substantially compressible web made from a blend containing an elastomeric material selected from natural rubber and a mixture of natural rubber and synthetic rubber and a cross-linking agent, the blend being expanded, cast and cured to form a web containing micropores so that the resulting web comprises an open cell structure having a thickness ranging from about 40 to about 150 mils, an alcohol porosity of from about 45 to about 90% and a compressibility of at least about 20%.
In another aspect, the invention provides a method for making a separator for an energy-cell which includes, blending an elastomeric material selected from natural rubber and a mixture of natural rubber and synthetic rubber with a cross-linking agent, expanding the blend, and then curing said blend to form a substantially homogeneous web containing micropores so that the resulting web comprises an open cell structure having a thickness ranging from about 40 to about 150 mils, an alcohol porosity of from about 45 to about 90% and a compressibility of at least about 20%.
Another aspect of the invention provides an essentially maintenance free lead-acid battery cell which includes a cell container made of a polymeric material, at least one positive electrolytic plate and at least one negative electrolytic plate disposed in the container, an acidic electrolyte solution and a microporous compressible separator compressed between the positive plate and negative plate, the separator consisting essentially of a substantially compressible web made from an elastomeric material selected from natural rubber and a mixture of natural rubber and synthetic rubber, the elastomeric material being blended with a cross-linking agent, the blend being expanded, cast and cured to form a substantially homogenous compressible web containing micropores so that the resulting web comprises an open cell structure having a thickness ranging from about 40 to about 150 mils, an alcohol porosity of from about 45 to about 90% and a compressibility of at least about 20%.
An advantage of the separators according to the invention is that the separators are readily compressible enabling easier cell assembly and the separators retain their resiliency over the life of the energy storage cell enabling longer life operation of energy cells, such as rechargeable sealed lead-acid battery cells containing the separators, particularly lead-acid battery cells operating on a gas recombination cycle. The separators made according to the invention also tend to attenuate the recombination reaction thus decreasing the tendency for a thermal runaway reaction or self-discharge of the negative plates.